This invention relates to semiconductor memory devices, and more particularly to reference voltage generator circuits for precharging dummy cells in dynamic memory devices.
Dynamic MOS read/write memory devices have been constructed generally as shown in U.S. Pat. No. 4,081,701 (a 16K dynamic ROM) issued to White, McAdams and Redwine, or U.S. Pat. No. 4,239,993 (a 64K dynamic RAM) issued to McAlexander, White and Rao, both assigned to Texas Instruments.
In these prior devices a row of dummy cells on each side of the differential sense amplifiers was used as a reference input. The capacitors in the dummy cells were about one-half the size of the storage capacitors, and these dummy capacitors were predischarged to ground. Thus, the signal produced in the bit lines by the dummy cells was about half way between that produced by a one and that produced by a zero in the storage cell.
When the cell size is reduced to the level needed to manufacture high density memory devices of 256K-bit or 1-Megabit and beyond, the problem of forming the dummy capacitors at the proper ratio to the storage capacitors becomes formidable, from a process standpoint. Also, at these high densities, it becomes necessary to use a precharge level for the bit lines at one-half the supply level, instead of a full supply level; this is because half of the bit lines always are at the one level and half at zero level after every active cycle, and the bit lines can charge each other to the one-half level internal to the chip instead of charging all back up to the supply by externally-supplied current.
It is the principal object of this invention to provide improved circuitry for high density dynamic RAM devices, particularly for dummy cell precharging. Another object is to provide dummy cell precharging circuitry for a CMOS dynamic RAM in which the dummy cell capacitors are the same size as the storage capacitors. A further object is to provide dummy cell precharging circuitry which is of high speed and low power dissipation. An additional object is to provide dummy cell precharge to a reference level less than half the supply level, compensated for changes in the supply, in which the reference level can be readily changed